Research Collection Doctoral Thesis Design of an experiment to search for invisible decays of ortho- positronium in vacuum and ortho-positronium formation studies in mesostructured silica films Author(s): Gendotti, Ulisse Publication Date: 2010 Permanent Link: https://doi.org/10.3929/ethz-a-006250397 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Diss. ETHZ No. 14774 and DISS. ETH NO. 19067 ETHZ-IPP Internal Report 2002-05 August 2002 Design of an experiment to search for invisible decays of ortho-positronium in vacuum and ortho-positronium formation studies in mesostructured silica films A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of Doctor of Sciences presented by Ulisse Gendotti Dipl. Phys., ETH Zurich born 12.12.1981 citizen of Prato Leventina, TI Switzerland accepted on recommendation of Prof. Andre Rubbia, examiner Prof. G¨unther Dissertori, co-examiner 2010 ii Abstract In this PhD thesis, the design of an experiment for the search for invisible decays of ortho-positronium (o-Ps) in vacuum is presented. Such decays could be interpreted as a signal for Mirror type Dark Matter. The goal is to reach a sensitivity in the branching ratio of Br(o Ps invisible) 10−7 to confront the annual modulation of the signal observed by DAMA/NaI− → and DAMA/LIBRA≃ (with 8.2σ significance) with Mirror Dark Matter scenarios. In case of a signal observation, the experiment would offer a unique and essential feature for a check of this signal: by changing the number of collisions of o-Ps in the vacuum cavity by a factor 2 the signal is scaled by the same factor while the background stays constant. In case of a null result, this search will provide an upper limit for the photon-mirror photon mixing strength about one order of magnitude better than presently derived from the Big Bang Nucleosynthesis. Based on the ETH Zurich slow positron beam, a special apparatus to select an appro- priate target for the Ps formation has been constructed. In some targets, fractions as high as 40 % of the positrons implanted in the film were converted into o-Ps and emitted from the surface. Moreover, measurements in a temperature range of 50-400 K were performed showing that the o-Ps yield is almost independent of the film temperature. Quantum mechanical effects related to confinement of o-Ps in the pore have been observed. In par- ticular, it has been experimentally demonstrated that the lowest possible emission energy of o-Ps is limited by the energy of its ground state in the pore confinement potential. These experimental results provide a solid ground for the future development of low temperature e+ o-Ps converters . → iii iv ABSTRACT Zusammenfassung In dieser Dissertation wird die Entwicklung und Planung f¨ur ein Experiment zur Suche nach unsichtbaren Zerf¨allen von Ortho-Positronium Atomen (o-Ps)im Vakuum vorgestellt. Solche Zerf¨alle k¨onnten als Signal f¨ur Dunkler Materie in einer Spiegelwelt interpretiert werden. Das Ziel ist es, eine Empfindlichkeit im Verzweigungsverh¨altnis von Br(o Ps invisible) 10−7 zu erreichen, um die von den DAMA/NaI und DAMA/LIBRA− Experi-→ ≃ menten beobachtete j¨ahrliche Variation ihres Signals (mit 8.2σ Signifikanz) mit Szenarien von Dunkler Materie in der Spiegelwelt vergleichen zu k¨onnen. Falls ein Signal beobachtet wird, bietet das vorgestellte Experiment eine einzigartige und wesentliche M¨oglichkeit, dieses Signal zu ¨uberpr¨ufen: indem man die Anzahl Kollisionen von o-Ps mit der Vaku- umkammer um einen Faktor 2 ¨andert, ¨andert auch das Signal um denselben Faktor bei gleich bleibendem Untergrund. Falls kein Signal beobachtet wird (Null-Resultat), liefert das Experiment eine obere Grenze f¨ur die St¨arke der Photon - Spiegelphoton Mischung, die etwa um eine Gr¨ossenornung besser ist, als was heute aus der Big Bang Nukleosynthese hergeleitet werden kann. Basierend auf dem Strahl von langsamen Positronen der ETH Z¨urich wurde ein spezieller Apparat konstruiert, um ein m¨oglichst gutes Target f¨ur die Positronium-Formation auszuw¨ahlen. In einigen Targets bildeten bis zu 40 % der im Film implantierten Positronen o-Ps, das von der Oberfl¨ache emittiert wurde. Zus¨atzlich haben Messungen im Temperaturbereich von 50 - 400 K ergeben, dass die o-Ps-Ausbeute nahezu unabh¨angig von der Filmtemper- atur ist. Quantenmechanische Effekte wurden beobachtet, die vom Einschluss der o-Ps- Atome in den Poren des Filmtargets herr¨uhren. Insbesondere wurde experimentell gezeigt, dass die niedrigste Energie der emittierten o-Ps-Atome limitert ist durch die Energie des Grundzustandas von o-Ps im Potential, das die Atome in den Poren einschliesst. Diese ex- perimentellen Resultate bieten eine solide Grundlage, f¨ur die Entwicklung von zuk¨unftigen e+ o-Ps Konvertern bei niedrigen Temperaturen. → v vi ZUSAMMENFASSUNG Contents Abstract iii Zusammenfassung v 1 Mirror Matter as Dark Matter 1 1.1 PositroniumandDarkMatter ......................... 2 1.2 Experimental evidence forMirrorDarkMatter . ... 7 2 Search for the invisible decay of o-Ps in Aereogel 9 2.1 Experimentaltechnique............................. 9 2.2 Background estimation and dedicated engineering run . .... 17 2.3 Dataanalysis .................................. 19 2.4 Results...................................... 20 2.5 Interpretation .................................. 23 3 Design of an experiment to search for invisible decays of o-Ps in vacuum 25 3.1 Thesetup .................................... 26 3.1.1 Theslowpositronbeam........................ 27 3.1.2 o-Psproductiontarget......................... 28 3.1.3 Thevacuumcavity ........................... 29 3.1.4 Positrontaggingsystem . .. .. 32 3.1.5 Trigger efficiency and confidence level . 34 3.1.6 Thephotondetector .......................... 39 3.2 Backgroundestimation ............................. 39 3.2.1 Fastbackscattered o-Ps fromthecarbonfoil . .. 43 3.2.2 Backscatteredpositrons . 43 3.3 Sensitivity.................................... 45 3.4 Summary .................................... 47 4 Study of positronium emission from porous silica films 55 4.1 Positrons and positronium in solids . 55 4.1.1 Positronium formation mechanism . 57 4.1.2 Thepick-offprocess........................... 58 vii viii CONTENTS 4.1.3 Positronium diffusion and thermalization in porous films . 59 4.2 Dopplerbroadeningspectroscopy(DBS) . .. 61 4.3 Angular correlation of annihilation radiation (ACAR)..................................... 61 4.4 3γ/2γ technique................................. 62 4.5 Positron annihilation lifetime spectroscopy (PALS) . .. 63 4.6 Positronium time-of-flight (Ps-TOF) . 63 5 The slow positron beam 65 5.1 Design and construction of a 22Na positronsourcechamber. 65 5.1.1 Thepositronmoderator ........................ 69 5.2 Thepositrontransportsystem. 71 5.3 Thevacuumsystem............................... 73 5.4 Positron and positronium tagging . 73 5.4.1 Pulsedbeammode ........................... 73 5.4.2 Continuos beam and secondary electron trigger mode . 74 5.5 Beam profile monitoring using a MCP with a position sensitive screen . .. 77 5.6 Conclusions ................................... 79 6 Design and construction of the PALS and TOF detectors 81 6.1 Monte-Carlosimulation............................. 81 6.2 The magnetic transport system in the target region . .... 82 6.3 ThePALSdetector ............................... 83 6.3.1 3γ/2γ detection efficiency correction using MC simulations . 84 6.4 The positronium time-of-flight detector . .. 87 6.5 Cooling of the target using a cryocooler . 89 6.5.1 Position calibration of the TOF spectrometer at low temperatures. 91 6.6 TheDAQsystem ................................ 92 6.7 PALSdataprocessing ............................. 92 6.8 TOFdataprocessing .............................. 93 6.9 Conclusions ................................... 96 7 Analysis of the PALS spectra 97 7.1 Samplepreparation............................... 97 7.2 Extrapolation of the pore size from the o-Ps lifetime . ... 99 7.2.1 Lifetime in sub-nanometer pores: Tau-Eldrup model . 99 7.2.2 Lifetime in mesoporous materials, the RTE model . 99 7.3 Determination of the lifetime in the pores . 101 7.3.1 Measured yield of the o-Ps emission into the vacuum and the o-Ps lifetimeinpores............................. 107 7.3.2 Calculation of the pore size . 110 7.4 Conclusions ................................... 111 CONTENTS ix 8 Analysis of the TOF data 113 8.1 Ortho-positronium thermalization in porous films . 114 8.1.1 Theclassicalmodel........................... 114 8.1.2 Positronium cooling by phonon scattering . 117 8.2 Determination of the o-Ps energy from the TOF measurements . ...... 120 8.2.1 Lifetime and detection efficiency correction and TOF peak analysis 121 8.2.2 Subtractionofthetargetcomponent . 123 8.2.3 Subtraction of the non-thermalized component . 124 8.3 Lowtemperaturemeasurements . 127 8.3.1 Comparison with the MC simulation and reconstruction of the total o-Psenergy ............................... 131 8.3.2 Ps in thermodynamic equilibrium at a temperature T in the pore . 134 8.3.3 TheP32sample............................. 137 8.3.4 Conclusions ............................... 138 9 Conclusions 141 10 Future perspectives 143 10.1 1S-2S transition of ortho-positronium . 143 10.2PsandAntihydrogen .............................. 144 10.2.1 Precise o-Ps decay rate measurement . 145 10.3 Bose-Einsteincondensationofo-Ps